1. Field of the Invention
This invention relates to a control system for an electric motor and, more particularly, to a control system for maintaining a desired spatial relationship between the movable member of the motor and the resultant magnetic field created by the stationary member of the motor.
2. Description of the Prior Art
Methods for achieving precise control of motor torque, speed or position are well known in the art of servo control systems. The use of AC motors and servo control systems therefor is also well known and is generally employed in instrument servo applications. Induction motors are often used in such applications and the control system therefor is designed to yield good servo performance. An example of such a servo control system is a multiphase AC carrier amplifier. The major disadvantage associated with the use of such AC motor servo controls is their high inefficiency due to the characteristics of the motor. Accordingly, such AC motor servo control systems are impractical at power levels above 100 to 200 watts of output power. If the AC induction motor is designed for high efficiency, the motor becomes nonlinear and cannot be controlled precisely and economically by servo control techniques.
Therefore, DC motors are usually chosen for applications where precise control of torque, speed or position is essential and power outputs above 100 to 200 watts are required. The manner of use of DC motors in such applications is a very well known and highly developed art. The use of DC motors in such applications with the appropriate controls and feedback transducers provides performance that is superior to that of AC motors and their corresponding controls in servo control applications and, therefore, DC motors are commonly used in such applications rather than AC motors.
The major disadvantages associated with the use of DC motors and control systems therefor result from the existence of brushes and commutators within the motor that mechanically control the orientation of the magnetic field within the motor. Such brushes and commutators are subject to wear and require periodic maintenance and replacement, which is costly to perform and requires a motor construction that makes the brushes and commutators accessible to maintenance personnel. Moreover, the movable members of most DC motors have conductors wound therearound through which electric current is passed to generate the magnetic field required to produce torque or force. The current passing through the windings generates heat within the movable member which is difficult to remove and limits the performance of the motor.
A servo motor control system for an AC synchronous reluctance motor is described in U.S. Pat. No. 3,737,747 issued to Krauer on June 5, 1973. That control system includes a three-phase power amplifier that is connected to the three-phase stator windings of a synchronous reluctance motor and drives the motor. Due to the nonlinearities and magnetic saturation of the motor and the limitations of the power amplifier, the system is limited in performance and does not achieve operating characteristics which are comparable to those achieved by DC motors and the control systems therefor.
Accordingly, there exists a need for a control system for a motor having no mechanical means integral therewith for commutating motor current which achieves performance that is equivalent to that achieved with control systems for motors having such means.